1. Field of the Invention
This invention relates to a method for producing simulated PVT (Pressure-Volume-Temperature) parameters from well log information without the need to collect physical samples from petroleum reservoirs. In particular the invention relates to a computing apparatus which accepts wireline logging pressure and temperature measurements as a function of borehole depth and generates simulated PVT parameters. More particularly, the invention concerns enhancing the statistical accuracy of the PVT parameters with knowledge of geochemical characteristics of fluids of a reservoir of which the well is drilled.
2. Description of the Prior Art
Petroleum fluids (liquids and gas) are found in geological reservoirs where they are contained at high pressure (relative to ambient atmospheric pressure), and usually also at an elevated temperature (relevant to ambient atmospheric temperature). At such pressures, the reservoir fluid initially exists as a single-phase fluid, but will release dissolved gas to form a two-phase fluid with separate gas and oil components if the reservoir fluid has its initial pressure sufficiently reduced toward ambient atmospheric pressure. Also, the initial relatively high temperature of the reservoir fluid results in volumetric contraction of a given mass of fluid as it cools toward ambient atmospheric temperature if withdrawn from the well.
When petroleum exploration wells are drilled and hydrocarbon fluids are found, a well fluid test is usually performed. This test usually involves flowing the well fluid to the surface, mutually separating the oil and the gas in a separator, separately measuring the oil and gas flow rates, and then flaring the products.
It is also desirable to take samples of the oil and gas for chemical and physical analysis. Such samples of reservoir fluid are collected as early as possible in the life of a reservoir, and are analyzed in specialist laboratories. The information which this provides is vital in the planning and development of petroleum fields and for assessing their viability and monitoring their performance.
There are two ways of collecting these samples:
1. Open-hole Bottom Hole Sampling of the fluid directly from the reservoir, and
2. Surface Recombination Sampling of the fluid at the surface.
In Open-hole Bottom Hole Sampling (BHS) a special sampling tool is run into the well to obtain a pressurized sample of the formation fluid present in the well bore. During sampling, the pressure of the sample is maintained at the same downhole pressure at which it is obtained from the formation surrounding the borehole. Provided the well pressure at the sampling depth is above the “Saturation Pressure” of the reservoir fluid, the sample will be a single-phase fluid representative of the reservoir fluid, i.e. an aliquot.
Surface Recombination Sampling (SRS) involves collecting separate oil and gas samples from the surface production facility (e.g. from the gas/oil separator). These samples are recombined in the correct proportions at the analytical laboratory to create a composite fluid which is intended to be representative of the reservoir fluid, i.e. a re-formed aliquot.
Several Open-hole BHS tools are currently available commercially, which function by a common principle of operation. These include Schlumberger's MDT tool, Baker Atlas' RCI tool and Halliburton's RDT tool. As a group these are often referred to as Wireline Formation Test tools (WFT). A plurality of samples can be collected (e.g. from different producing zones) from one trip into the well.
A typical WFT tool is run into the well to tap a sample of reservoir fluid at the required depth by controlled opening of an internal chamber to admit reservoir fluid, followed by sealing of the sample-holding chamber after admission of predetermined volume of fluid. The tool is then retrieved from the well and the sample is transferred from the tool for shipment to the analytical laboratory. The downhole PVT characteristics of each sample is then determined.
Wireline Formation Test tools provide not only PVT quality samples at most promising intervals of the borehole, but also a pressure gradient and temperature profile log of the well. A pressure gradient is used to determine fluid contact level, formation fluid density and completion strategies. As mentioned above, the fluid samples collected by WFT tools are sent to analytical laboratories for PVT (pressure-volume-temperature) measurements.
Current well logging practice measures pressure profiles in multiple potential hydrocarbon producing zones and collects PVT quality samples in only the most promising intervals. The number of samples collected is limited because of:                1. The rig-time (cost) associated with running the wireline formation tester;        2. The time required to reduce the presence of oil-based drilling mud which contaminates samples and alters their PVT properties; and        3. The number of available chambers in the tool for sample collection.        
In the past, there has been no convenient system or method by which the pressure profile and temperature profile measurements from the WFT log, for example, can be used to predict the PVT characteristics obtained from the samples obtained while obtaining the WFT log.
3. Identification of Objects of the Invention
A primary object of the invention is to provide a computer based analytical tool and method to produce PVT characteristics of petroleum reservoir fluids as a function of depth from pressure and temperature profile measurements from a logging tool without the need for collection of fluid samples.
Another object of the invention is to provide a computer based analytical tool and method to produce PVT characteristics of petroleum reservoir fluids as a function of depth from log measurements of formation pressure, temperature and pressure gradient derived from the formation pressure.
Another object of the invention is to provide PVT characteristics of reservoir fluids from logging tool measurements of reservoir pressure, reservoir temperature, and pressure gradient with information as to the physical location of the well which enables pertinent geochemical parameters to be identified for enhanced statistical accuracy of the PVT characteristics.